In the continuous casting of steel, molten steel charged into a tundish is poured into a mold for continuous casting through an immersion nozzle connected to a bottom portion of the tundish. In this case, a molten steel flow discharged into the inside of the mold from discharge holes of the immersion nozzle includes non-metallic inclusions such as alumina clusters and bubbles of an inert gas which is blown off from an inner wall surface of an upper nozzle (an inert gas blown off for preventing the clogging of the nozzle caused by adhesion or stacking of alumina or the like). When these non-metallic inclusions and bubbles are caught in a solidified shell, product defects (inclusion-caused defects, bubble-caused defects) occur. Further, a mold flux (mold powder) is entrained into an upward molten steel flow which reaches a meniscus so that the mold flux is also caught in the solidified shell leading to defects of a product.
Conventionally, to prevent the non-metallic inclusions, mold flux and bubbles in the molten steel from causing a product defect being caught in the solidified shell, a magnetic field is applied to a molten steel flow in the mold and the flow of molten steel is controlled by making use of an electromagnetic force generated by a magnetic field. Many proposals have been made with respect to this technique.
For example, Patent Document 1 discloses a method where a molten steel flow is braked by a DC current magnetic field which is applied to a pair of upper magnetic poles which is arranged to face each other with a mold long-side portion sandwiched therebetween and a pair of lower magnetic poles which is arranged to face each other with the mold long-side portion sandwiched therebetween. In this method, out of the molten steel flow which is divided into an upward flow and a downward flow after being discharged from discharge openings of the immersion nozzle, the downward flow is braked by a lower DC magnetic field and the upward flow is braked by an upper DC magnetic field thus preventing the non-metallic inclusions and a mold flux included in the molten steel flow from being caught in the solidified shell.
Patent document 2 discloses a method where a molten steel flow is braked by a DC current magnetic field which is applied to a pair of upper magnetic poles which is arranged to face each other with a mold long-side portion sandwiched therebetween and a pair of lower magnetic poles which is arranged to face each other with the mold long-side portion sandwiched therebetween in the same manner as Patent Document 1, and an AC magnetic field is applied to the upper magnetic poles or the lower magnetic poles in a superimposed manner. This method provides the braking of the molten steel flow by the DC magnetic field in the same manner as Patent Document 1, and also aims at the acquisition of a cleaning effect of non-metallic inclusions or the like on an interface of a solidified shell due to stirring of molten steel by an AC magnetic field.
Further, Patent Document 3 discloses a method where a molten steel flow is braked by a DC magnetic field which is applied to a pair of upper magnetic poles which is arranged so that the magnetic poles face each other with a mold long-side portion sandwiched therebetween and a pair of lower magnetic poles which is arranged so that the magnetic poles face each other with the mold long-side portion sandwiched therebetween respectively. In this method, the intensity of DC magnetic field and an intensity ratio between a DC magnetic field of the upper magnetic poles and a DC magnetic field of the lower magnetic poles are set to values which fall within specific numerical ranges.
Further, Patent Document 4, 5 disclose a continuous casting method where the catching of bubbles in a solidified shell can be suppressed by controlling a surface tension due to concentration gradient of C, S, N, O in molten steel on a front surface of the solidified shell, that is, by adjusting the concentrations of C, S, N, O in molten steel such that the surface tension becomes equal to or below a predetermined value.